The present invention generally relates to fully differential amplifiers and more particularly to an improved structure that utilizes a switching block having binary weighted field effect transistors to modify the amplifier gain and to provide that the gain control is largely insensitive to manufacturing variations and temperature changes and to maintain the high speed and stability of the amplifier, which would otherwise be compromised if a different gain adjust technique had been employed.
An operational amplifier is a relatively high gain amplifier capable of being used in various kinds of feedback circuits for performing certain mathematical operations. For example, operational amplifier circuits can provide programmable gain, signal summation, integration, and differentiation, and various other useful functions too numerous to mention here. One of the most popular configurations of the operational amplifier is to provide fixed gain amplification (e.g., a gain of 10xc3x97) with the use of resistors in a feedback network. If the operational amplifier gain is high enough, then the overall gain with feedback is only determined by the ratio of resistors. Traditionally, an operational amplifier has one differential input and a single output. A fully differential operational amplifier is similar to traditional operational amplifier except that it has a differential output as well as a differential input. It too, can be configured as a feedback amplifier with the use of resistors to provide a fixed gain that is dependent on the ratio of resistors. Fully differential feedback amplifiers are widely used in modern integrated circuits because they have advantages over their single-ended counterparts including a larger output voltage swing and less susceptibility to common mode noise. There are countless applications for fully differential feedback amplifiers, such as in analog to digital converters (ADCs) and digital to analog converters (DACs). Two examples of fully differential feedback amplifiers are shown in FIGS. 1 and 2. In FIG. 1, the differential voltage gain is simply R3/R1 if the amplifier, represented by the triangular symbol 25, has a very high gain. More specifically,                                           V            od                                V            sd                          =                              (                                          R                3                                            R                1                                      )                    =          gain                                    [t1]            
Thus, the overall gain is the dependent on a ratio of resistors, which should be independent of temperature and global process variations if the resistors are all of the same type and vary with temperature and process the same way. FIG. 2 shows a different feedback amplifier configuration. The amplifier""s 26 gain is (1+(RB/RA)) and thus it is also dependent on the ratio of resistors. More specifically,                                           V            od                                V            sd                          =                  1          +                                    R              B                                      R              A                                                          [t2]            
Although the gain in the amplifiers 25, 26 discussed above is independent of global process and temperature changes, it is subject to mismatch between the resistors. For example, consider the case of using the circuit of FIG. 1 configured to have a gain of two by making R3=2* R1. If there is process mismatch between R3 and R1, causing R3=2.1*R1, then the overall gain will be 2.1. Other second order effects can also cause non-ideal gains, such as low operational amplifier gain. For this reason, in applications that require a very precise gain, there is a need to adjust or calibrate the gain. Gain modification is also useful to make up for shortcomings in other amplifiers in the signal path. One application where this is used is in an ADC where an incoming signal is analog and must be amplified a precise amount before becoming digitized. Any error in this amplification translates to an overall gain error in the ADC.
One way to adjust the gain in a fully differential feedback amplifier such as that in FIG. 1, is to make the resistors adjustable. This can be done by replacing the resistors with FETs whose resistance can be varied by their gate voltage or by switching in parallel devices. Alternately, the resistors can be maintained, but parallel resistors can be switched in or out to effectively change their resistance. However, adding the necessary switches or controls to connect/disconnect the resistors or by using FETs as resistors, can reduce the quality of the amplifier, slowing it down, or making it unstable. The invention disclosure below details an improved differential feedback amplifier that has gain control yet largely remains insensitive to manufacturing variations and temperature changes and does this with no degradation in speed or stability. It also describes a means by which the fully differential feedback amplifier speed can be improved by using strategically placed buffers that are scaled in a manor that does not impact the circuit gain.
In view of the foregoing and other problems, disadvantages, and drawbacks of the conventional differential amplifiers, the present invention has been devised, and it is an object of the present invention to provide a structure and method for an improved differential amplifier that has gain control that is largely insensitive to manufacturing variations and temperature changes and maintains or increases the bandwidth that the amplifier would have had without the gain control.
In order to attain the object(s) suggested above, there is provided, according to one aspect of the invention, an integrated circuit which includes a differential amplifier that has at least two inputs and at least two outputs. The invention also includes a pair of first resistors, each of which is coupled to one of the inputs; and a pair of first source followers, each of which is coupled to one of the first resistors. Further, the invention includes a pair of second source followers, each of which is coupled to one of the outputs; a pair of second resistors, each of which is coupled to one of the second source followers and to one of the inputs; and a gain device connected between the first resistors and the first source followers. The second source followers and the second resistors make up a feedback loop for the differential amplifier. The first and second pairs of fixed resistors as well as the gain device control the gain of the integrated circuit chip which remains largely insensitive to manufacturing process variations and temperature changes.
The gain device comprises a field effect transistor switching block. The field effect transistor switching block includes binary weighted resistive switches. The first source followers are scaled with respect to the second source followers in the same relationship that the second resistors are scaled to the first resistors. The integrated circuit chip optionally includes a pair of third source followers, each of which is connected to one of the outputs and to one of the second source followers. This third pair of source followers is used to buffer the differential amplifier output from any load that may be connected to the output in order to increase the speed of the resulting amplifier when heavily loaded.
This invention provides fully differential feedback amplifier that has the benefits of insensitivity to temperature and process variations and with a means to adjust or calibrate the gain. Also, through the use of the source followers in the input and feedback paths, the speed performance is enhanced over designs that do not contain these. These benefits are especially useful in high speed applications that require high analog voltage accuracy such as a high speed analog to digital converters or digital to analog converters.